Products of bonded refractory particles are commonly made by a press in which a charge of a mixture of the particles and a resinous binder and usually known fillers is compacted into a desired shape between ram-actuated mating mold parts of a press. The press action is usually attended by heat, such as steam heating of the molds, in order to set or harden the binder which for this reason must be thermosetting. U.S. Pat. No. 3,268,632 to Eves illustrates a press of this general type for molding grinding wheels assemblies.
With molding systems presently in use, there are several disadvantages, both economically and in the quality of the products or bodies molded. In the usual process, a charge of the mixture of refractory particles and binder must be heated uniformly throughout the charge to insure fluidity of the binder. Compaction of the charge beings early in the heating stage, but completion of the compaction, when maximum density is obtained, must be postponed until the charge is well heated and the fluidized binder has wetted well the refractory particles. Since on a time basis, the heating step consumes a substantial portion of the press cycle, the result is that relatively expensive press equipment is used primarily only for heating. Consequently, the rate of production of the press is seriously hampered.
The practice of compacting or compressing a charge of particles and binder while the charge is being heated, first to fluidize the resin and then to set or harden it, culminating eventually in a maximum compression has been found actually to introduce fine cracks and fractures along the outer surfaces of the product or body molded. These defects weaken the strength of the molded product and also shorten its useful life.